In the 5th generation, 5G, wireless systems in standardisation, beamforming and Multiple Input Multiple Output. MIMO, transmission will be central technologies. Increasing capacity requirements is driving this development where more and more MIMO transmission is introduced in existing frequency bands. However, this will soon become insufficient, thereby requiring migration into spectrum at higher carrier frequencies, starting at 3.5-5 GHz, continuing to the soon available 28 GHz band and beyond, towards 60 GHz. For these higher bands, beamforming with massive antenna arrays, in the end with hundreds of elements, will be needed to compensate for the worsening radio propagation. At these higher frequencies cost, size and power constraints will also prevent the AD/DA (Analogue Digital/Digital Analogue) conversion towards individual antenna elements, thereby most probably restricting solutions to so called analogue beamforming, or possibly hybrid beamforming with a few signal layers (perhaps 2-8) that can be used for multi-user access (over time and frequency).
At the same time the dominating multi-user access technology for 5G is expected to become some variant of Orthogonal Frequency Division Multiple Access, OFDM. As is well known, this access is associated with a resource grid, divided in time and frequency, see FIG. 2a. When multi layered (MIMO) transmission is used, there is one overlaid resource grid per layer, separated by spatial pre-coding.
With analogue beamforming there is only a limited number of digital signal chains that may use pre-coding, one per antenna port (assuming the number of antenna ports being equal to the number of layers). The advantage is that the signals to the many more antenna elements are then distributed by analogue signals (or at least not individually pre-coded signals). As a consequence the AD/DA and the internal interface capacities needs are minimised. A disadvantage may be that the beam steering is done by adjusting phase and possible amplitude weights before the antenna elements, thereby setting up a fixed beam pattern that remains valid during the whole symbol. If this beam is “narrow”, which it is has to be to reach maximum throughput and/or to counter the propagation effects to reach cell-edge wireless devices at high carrier frequencies, this means that the transmitted signal energy can only be directed in one direction, per symbol time (assuming 1 layer transmission). Since also pre-coded wireless devices using additional layers would be attenuated by this beam unless they are aligned to it, the consequence is that pure high gain analogue beam forming is restricted to single, or a few wireless device scheduling, per symbol time.
Different applications have very different needs when it comes to the transmission rate. Voice traffic e.g. requires <1 kbit/20 ms, whereas video download has a more or less unlimited bit rate need. Therefore, to avoid wasted capacity it is essential that the number of sub-carriers and the symbol time allows a fine enough granularity in terms of the total number of bits when combined.
The 3rd Generation Partnership Project, 3GPP, 5G standardisation seems to become based on a re-scaling of the 4G LTE resource grid, at least to some extent. This interface has a maximum bit rate per OFDM symbol of roughly 100000 bits, which is about 100 times larger than what is needed for RRC signalling, Transport Control Protocol, TOP, acknowledgement, ACK, or voice services. In this case most of the available resources would be wasted when applying analogue beam forming, see FIG. 2b. Recently a finer time division has relaxed this waste to some extent, however a fine time granularity is usually coupled to a short latency which in turn requires quicker computations that drive the HW requirements. Thus the problem addressed by the invention persists.